The invention relates to a drive circuit for a solenoid pump of a type controlling a small flow and in which a solenoid coil is energized intermittently to cause a reciprocatory motion of a piston to achieve a fluid supply.
Delivery and control of a fuel flow to a room heater usually takes place either through the use of a solenoid pump or through the combination of the use of a fluid head to deliver the fuel and a flow control valve which controls the flow rate. In the former arrangement, a pulse from an oscillator may be fed to the solenoid pump for control purposes, but the control circuit required to control the flow rate is complex. In the latter, the flow control valve of a mechanical type must be equipped with an orifice of a greatly reduced size in order to permit a control over a small flow rate, presenting difficulties in its manufacture and maintenance of the required precision.
To accommodate for this situation, there is provided a drive circuit for a solenoid pump which is simple in arrangement and which facilitates flow control. Such an arrangement is illustrated in FIG. 1, where an a.c. source A.C. is shown connected with one end of a series combination of a rectifier SR1 and resistor R1, the other end of which is connected with one end of a solenoid coil 1 of a solenoid pump. The other end of the coil 1 is connected with a thyristor SCR1, which is in turn connected with the other terminal of the a.c. source. A capacitor C1 is connected in shunt with the series combination of coil 1 and SCR1. The gate of thyristor SCR1 is connected with a shunt resistor R4, the other end of which is connected with the other terminal of the source. The gate is also connected with a trigger circuit which comprises a variable resistor VR1, capacitor C2 and trigger diode TD. A diode SR2 is connected through current adjusting resistors R2 and R3 with the trigger circuit, and prevents a reverse flow of the charge on the capacitor C2 to the source. A Zener diode ZD1 is connected at its one end with the junction between the resistors R2, R3 and with the other terminal of the source at its other end, and functions to supply a constant voltage to the trigger circuit. A capacitor C3 may be connected in shunt with the Zener diode for maintaining the constant voltage. If desired, the resistor R1 may be formed by a variable resistor.
In operation, the alternating current from the source is rectified by rectifier SR1 into a d.c. current, which is passed through the resistor R1 to the capacitor C1. The capacitor C1 may be charged to the peak value of the a.c. source in a time interval T.sub.CM which is determined by the resistance of resistor R1 and the capacitance of capacitor C1. Such time interval may be several a.c. cycles. In the trigger circuit, the capacitor C2 is charged until the voltage thereacross reaches a threshold voltage of the trigger diode TD, whereupon diode TD conducts to cause the capacitor C2 to discharge through the resistor R4, thereby developing a trigger pulse which is applied to the gate of the thyristor SCR1. This trigger pulse occurs repeatedly with a period Ttr which depends on the breakdown voltage of the Zener diode, resistance of resistors R3, VR1, capacitance of capacitor C2 and the threshold voltage of the trigger diode TD. Thus, the period Ttr can be varied by the adjustment of the variable resistor VR1.
In response to the trigger pulse, the thyristor SCR1 conducts, whereby the capacitor C1 discharges through the solenoid 1. Current ceases to flow through the solenoid 1 during the negative half cycle of the source, rendering the thyristor SCR1 non-conductive. The solenoid 1 can be energized by a discharge current of the capacitor C1 after it has been charged to the peak value of the source, if the number of energizations per minute of the solenoid or the interval Ttr is adjusted by means of the variable resistor VR1, such that T.sub.CM &lt;Ttr.
The conventional drive circuit described above is simple in arrangement and provides a desired operating characteristic while reducing the number of parts required. It is to be noted that when operated with a commercial frequency, the solenoid pump will achieve a discharge performance which is generally greater than is desired. In order to reduce the frequency, an oscillator employing a pair of thyristors is often provided which is d.c. operated by conversion from the source of a commercial frequency. As compared with such an arrangement, it will be seen that the described circuit requires a single thyristor and hence a single trigger circuit, thus simplifying the circuit arrangement and avoiding the need for the provision of a high capacity d.c. source. The flow rate can be controlled by changing the period of the trigger pulse, which is conveniently accomplished by adjusting the variable resistor VR1.
FIG. 2a shows the mechanical construction of an exemplary solenoid pump. Specifically, it includes a hollow core 8 carrying a valve body 9 and disposed slidably inside the solenoid 1 so as to be excited by the latter. The pump also includes a pair of permanent magnets 4, 5 disposed in axial alignment on opposite sides of the core with the opposite magnetic poles thereof in opposing relationship, and a pair of springs 6, 7 disposed between the respective magnets and the core. A check valve 10 is included in the inlet passage of the pump. The pump is shown in longitudinal section in FIG. 2a, while FIGS. 2a and 2b show schematically the pump when the solenoid 1 is energized and de-energized, respectively. When the solenoid 1 is energized, the core 8 is magnetized to the polarity shown, and experiences an attraction by the magnet 4 and a repulsion by the magnet 5, thus moving upward as shown in FIG. 2b. When the solenoid 1 is de-energized, the core 8 would tend to maintain its position shown in FIG. 2b, but the spring 6 urges it downwardly toward the magnet 5, until it reaches a neutral position shown in FIG. 2c where the resilience of the springs 6, 7 is balanced.
In the prior art drive circuit, it will be understood that the solenoid 1 will be energized from the a.c. source directly in addition to the energization by the discharge of the capacitor C1 upon conduction of the thyristor SCR1, since it is in series circuit relationship with the source. Because the solenoid 1 includes a certain resistance, the capacitor C1 will be fed with a charging current during its discharge. This results in a failure of the thyristor SCR1 to turn off at the zero crossover of the source voltage. This in turn requires the choice of a relatively high resistance for the resistor R1, which, however, prevents a rapid charging of the capacitor C1. As a result, a limit is imposed on the achievable range of the adjustment of the energization cycle of the solenoid 1 and hence the drive rate of the pump by the variable resistor VR1.
In addition, a fluctuation in the source voltage results in a variation in the energy stored on or discharged from the capacitor C1, leading to a varying discharge per unit time of the solenoid pump.